1. Field of the Invention
The present invention relates to methods and apparatus for reducing the rate of heat transfer across an interface forming a common boundary for a fluid body and another body so as to reduce unwanted heat loss or heat gain from one body to another. Some aspects of the present invention relate more particularly to reducing heat transfer through windows or panels separating fluid bodies of differing temperatures.
2. Description of the Prior Art
Previous attempts to control heat transfer between adjacent bodies have most often employed heat insulating materials. Thermal insulation has also been accomplished by double wall construction having a vacuum therebetween. Likewise, multiple walls have been used to define spaces for "dead" (confined) air and/or other gases, or to provide paths for circulating various fluids to absorb and carry away a portion of the heat.
In the case of windows where light transmissibility must be maintained, utilization of low thermal conductivity materials as heat insulation is effectively precluded. The heat conductivity of glass and other transparent materials most commonly used for windows is not particularly high, but the path length is usually quite short. Accordingly, reduction of heat transfer through windows is particularly desirable. However, light transmissibility and transparency requirements heretofore have necessitated use of multiple pane/dead air, vacuum or removable fluid construction. For instance, it has been proposed to provide a reversible window having double pane, dead air construction with a third pane spaced therefrom and passages for entry of air at the bottom, and exit of air at the top, of the third pane from the space between the double panes and the third pane (See U.S. Pat. No. 3,925,945). In winter, the third pane faces the interior of the building, and in summer it faces the exterior. In this construction, natural convection between the double pane and the third pane is intended to absorb heat energy and conduct it to the outside air in summer, or into the building in the winter.
A typical dual pane window having a vacuum therebetween is shown in U.S. Pat. No. 3,990,201, and a multi-pane window having provision for flushing out the dead air space to eliminate moisture and condensation therein is shown in U.S. Pat. No. 3,932,971. A double pane window having water pumped through the space between the panes is shown in U.S. Pat. No. 4,024,726. A triple pane heat insulating window having a vacuum in one space between panes, and a forced flow of cooling fluid in the other space, is disclosed in U.S. Pat. No. 3,192,575. It therefore has been understood that unwanted heat transfer through windows can be reduced by adding more layers of glass at proper spacing, but this is quite expensive, especially for retrofit.
In winter, most of the heat passing through the window pane is lost to the air and this loss is greatly increased by natural convection. Air near the cold side of the window is heated by the glass, which reduces the density of such air, causing it to be forced upwardly by more dense air, which in turn approaches the pane, is heated, and the cycle repeats. Air near the warm side of a window is cooled and its density increases. The cool, dense air is pulled downwardly by gravity and warmer air replaces it. Again, this effect is repeated continually.
The flow of heat must be accounted for. When the outside air is cold and the inside air is warm, compensation for heat flow is accomplished by space heating. When the outside is warmer than the inside, heat flow compensation is accomplished by air conditioning.